Question

Aqueous solution of sodium chloride boils at higher temperature than water, but it freezes at lower temperature than water. Explain with reason.

Hint:

Non-volatile NaCl lowers the vapour pressure; this raises boiling point \( (\Delta T_b=iK_b m) \) and lowers freezing point \( (\Delta T_f=iK_f m) \), with \( i \approx 2 \).

Answer 1

Step 1: Sodium chloride is a non-volatile solute. When it is dissolved in water it lowers the vapour pressure of the solution below that of pure water. Elevation of boiling point and depression of freezing point are colligative properties that follow directly from this vapour-pressure lowering.
Step 2 (boils higher): A liquid boils when its vapour pressure equals the external (atmospheric) pressure. Because the salt solution has a lower vapour pressure, it must be heated to a higher temperature to reach atmospheric pressure. Hence the boiling point rises: elevation of boiling point \( \Delta T_b = i\,K_b\,m \).
Step 3 (freezes lower): Freezing occurs when the vapour pressure of the liquid equals that of the solid solvent (ice). The lowered vapour pressure of the solution meets the ice curve only at a lower temperature, so the solution freezes below 0 degree C: depression of freezing point \( \Delta T_f = i\,K_f\,m \).
Step 4: NaCl also ionises completely into Na⁺ and Cl^-, so the van't Hoff factor \( i \approx 2 \). This doubles the number of particles and makes both effects even larger. Therefore the NaCl solution boils higher and freezes lower than pure water.